1. Field of the Invention
The present invention generally relates to the field of optics and particularly to the field of prismatic optical devices. More specifically, the present invention pertains to optical devices such as telescopes and binoculars employing prismatic image erecting systems. Accordingly, the general objects of the present invention are to provide novel and improved apparatus of such character.
2. Description of the Related Art
Optical devices employing image erecting systems are well known in the art. Such devices typically utilize an objective and an ocular in cooperation with the erecting system to present a magnified image of an object to a viewer. Astronomical telescopes, terrestrial telescopes and prism binoculars are but a few examples of such devices. Image erecting systems are needed in such devices to reverse and invert, i.e., rectify in directions both parallel to and perpendicular to, the image formed by the objective since the objective inherently reverses and inverts images as light passes therethrough. However, the use of image erecting systems necessarily yields the undesirable result that four times the focal length of the erecting lens must be added to the sum of the focal lengths of the objective and ocular in order to present a sharp image to a user's eye. In the case of terrestrial telescopes this results in an unduly long draw tube.
This deficiency has conventionally been overcome in a number of ways, all of which employ the phenomenon of total internal reflection to reflect light rays passing therethrough to reverse and invert the image produced by the objective. According to a first solution, the image erecting system can be comprised of a pair of 45.degree.-45.degree.-90.degree. triangular prisms (hereinafter "Porro prisms") wherein the hypotenuse of each triangular prism faces one another and wherein the prisms are oriented at right angles to one another. This erecting system configuration is known as a "Porro prism erecting system." The deficiencies associated with such Porro prism erecting systems include the fact that each of the Porro prisms used therein have, in the past, been formed of a separate piece of expensive Crown glass with a refractive index larger than 1.5 using an expensive three step process of blocking, grinding and polishing. Further, in an attempt to minimize manufacturing costs, the size and shape of the prisms are often not optimized to the dimensions of the light ray cone expectedly incident thereto. This results in the existence of non-ray path areas of the prisms which serve no useful purpose but do have the undesirable effect of enlarging the overall size of the optical device.
Another deficiency of the Porro prism erecting systems of the related art arise from the need to mechanically mount the prisms within the optical device. Such mounting inevitably results in, at least, a small gap existing between the two prisms. This gap is responsible for light transmission losses (usually of about 8%) and polarization, chromatic and spherical aberrations at the surfaces of both hypotenuses. While polarization aberration and light reflection losses can be reduced by depositing expensive optical coatings between the prisms, this further increases costs. Furthermore, chromatic and spherical aberrations cannot be so rectified. Thus, additional lenses have been required to correct such defects. Finally, even assuming the presence of such a gap can be tolerated, the need to ensure precise alignment between the Porro prisms of the erecting system further increases the cost and difficulty of assembling such optical devices.
A second way to avoid the deficiency of utilizing an unduly long draw tube in an optical device employing an image erecting system is to form the image erecting system from a pair of pentangular roof prisms, also known as Amici prisms or ridge glass prisms. Aside from possessing most of the deficiencies noted above, the use of such image erecting systems entails bearing the additional expense of utilizing expensive jigs and other tools to manufacture the odd-shaped roof prisms and incurring additional assembly costs.
A final way to avoid the deficiency of utilizing an unduly long draw tube in such optical devices is to form the image erecting system from reflecting mirrors which are arranged in a prism-like form. While this solution can be somewhat less expensive than the above-noted solutions (due to the relatively low cost associated with producing the mirrors), any mirror misalignment which may exist will introduce errors the magnitude of which are multiplied up to four times as light passes through the erecting system and into the ocular. Thus, even small positional errors can lead to significant image distortion.
Therefore, there remains a need in the art for an optical system with an improved image erecting system which reduces light transmission losses as well as polarization, chromatic and spherical aberrations thereby minimizing the number of lenses necessary to form a high quality optical device.
There remains an additional need in the art for an optical system with an improved image erecting system which offers improved performance while reducing material, component-manufacturing and assembly costs.
There remains a further need in the art for an optical system with an improved image erecting system which offers an optimal combination of simplicity, economy, precision and versatility.